1. Field of the Invention
The present invention relates to an exhaust passage structure for an outboard motor.
2. Description of the Related Art Among engines for outboard motors, there is one that has a cylinder block comprising a plurality of cylinders arranged vertically. A cylinder head joined with the cylinder block has an intake port and an exhaust port being formed for each of the cylinders. Exhaust gas from each exhaust port is collected into a common exhaust passage, and is discharged out of the outboard motor.
A conventional means for collecting the exhaust gas discharged from each of the exhaust ports, for example, is disclosed in Japanese Laid-open Patent Publication No. Hei-6-16187, wherein the exhaust passages communicating to the exhaust ports in the cylinder head are formed integrally in the cylinder block. Another typical means for collecting exhaust gas discharged from respective exhaust ports is disclosed in Japanese Laid-open Utility Model Application No. Hei-4-134626, wherein a separate exhaust manifold is disposed between a cylinder head and an oil pan having exhaust passages formed therein.
However, forming the exhaust passages integrally in the cylinder block increases the size and weight of the cylinder block, and makes the configuration more complicated, thus leading to higher manufacturing cost. Also, since the exhaust passages are formed in greater proximity to the cylinder, the cylinder may be subjected to thermal deformation due to exhaust heat which should be avoided. Moreover, since the cylinder and the exhaust passage use a common coolant water jacket, it is impossible to control the temperature of the exhaust passage independently, thus resulting in lower cooling efficiency of the exhaust passage.
Although these problems may be solved by using an exhaust manifold, it is difficult to control the accuracy of assembly unless the top end and bottom end of the exhaust manifold lie on the same plane. Consequently, there may occur such undesirable effects as a lower yield in the machining processes.
In the case of a four-cylinder engine, for example, exhaust passages are arranged in such an order in the exhaust manifold where cylinder No. 1 may be at the top, followed by No. 2, No. 3, and No. 4, downward, while combustion in a four-cycle, four-cylinder engine generally occurs in the order of cylinder No. 1, No. 3, No. 4, and No. 2. As a result, exhaust emission interference occurs when the exhaust passages from cylinders No. 3 and No. 4, where combustion occurs consecutively are disposed adjacent to each other, and the exhaust passages from cylinders No. 2 and No. 1, where combustion occurs consecutively are disposed adjacent to each other, thus giving rise to a possibility of lowering engine performance.
Although these problems, for example, may be solved by a method extending the exhaust passages to below the engine and joining them at this position as disclosed in Japanese Laid-open Patent Publication No. Hei-9-49425, this makes the exhaust passages too long, resulting in a large size of the engine. Also the number of component parts increases and the construction becomes complex, while making it difficult to form the coolant water jacket and the coolant water passage.